RAID systems have a backup unit (battery) in case that a power outage stops the supply of power to the RAID systems. When a power outage occurs, such a RAID system supplies power from the backup unit to a volatile memory provided in a controller module so as to protect data stored in the volatile memory. Until power to the RAID system is restored, the RAID system continues to supply power from the backup unit to the volatile memory. As a result, the volatile memory continues to store data.
However, since the power capacity of the backup unit is limited, the data guarantee of the volatile memory depends on the electric capacity of the backup unit. If the capacity of the backup unit is increased, the reliability of the data guarantee of the volatile memory can be increased. However, the increase in the number of backup units increases the cost and size of the RAID system. Furthermore, after power has been restored, since it is impossible to ensure the backup of data until the backup unit is recharged, a write-through state is set in the RAID system. As a result, the processing power of the RAID system is significantly reduced. The write-through state is a state in which, when the controller module has successfully written data into a disk, the controller module notifies a host computer of the completion of writing of the data in the disk.
The following patent documents describe backup control for a RAID system.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2005-332471
[Patent Document 2] Japanese Laid-open Patent Publication No. 6-309234
[Patent Document 3] Japanese Laid-open Patent Publication No. 2000-357059
[Patent Document 4] Japanese Laid-open Patent Publication No. 8-220198